As the exhaustion of available frequencies continues to approach, there have been increasingly various efforts to enhance use efficiency of the frequencies. A method in which different systems share one frequency by any system detecting (Cognitive) a time and place not used by existing systems and carefully performing transmission and reception not to obstruct the existing systems, as in cognitive radio (see Non-Patent Document 1), has been considered as one approach.
However, the cognitive radio requires very high accuracy in detecting the existing system, and does not actually resolve an issue of interference between the existing system and other systems. Here, as a method of resolving the interference issue, there is a method of suppressing interference by controlling the directivity of an array antenna (see Non-Patent Documents 2 and 3).
For example, FIG. 43 is a diagram for explaining a conventional method of suppressing interference in an array antenna. In FIG. 43, a reception station 1121 includes an array antenna consisting of two antennas 1131 and 1132. The reception station 1121 simultaneously receives an interference signal U from an interference station 1011 and a desired signal D from a transmission station 1012.
In this case, a control circuit 1122 in the reception station 1121 assigns a weight to an array antenna phase from received signals X1 and X2 obtained by summing the desired signal D and the interference signal U received using the respective antennas 1131 and 1132 or from the interference signal U, so that a null in an interference direction viewed from the reception station 1121 and a high directivity in a desired signal direction are obtained.
The control circuit 1122 calculates a weight coefficient (weight) for forming an optimal directivity pattern P40 using an adaptation algorithm. For example, the adaptation algorithm is a simple method with excellent effects, and a power inversion adaptive array antenna (PIAA antenna) scheme is used (e.g., see Non-Patent Document 2). The power inversion adaptive array antenna is effective when there is an interference wave stronger than a desired wave, and an output signal that an inversion of an S/N ratio of the desired wave and the interference wave is obtained. In addition, a reference wave is unnecessary and an arrival direction of the desired wave does not have to be known. An algorithm for the power inversion adaptive array antenna will be described below.
Through the power inversion adaptive array antenna (PIAA antenna), it is possible to obtain a null in an interference direction viewed from the reception station 1121 and a high directivity in a direction of the desired signal.
The directivity of the array antenna is controlled so that the null in the interference direction and the high directivity in a desired wave direction are obtained to suppress the interference. However, since there is a trade-off relationship between an interference suppression effect and an antenna scale, and a hardware scale of the array antenna greatly affects the scale and price of the device, it is difficult to apply the array antenna to small portable terminals.
As described above, the directivity of the array antenna is controlled so that the null in the interference direction and the high directivity in the desired wave direction are obtained to suppress the interference. However, since there is the trade-off relationship between the interference suppression effect and the antenna scale, and the hardware scale of the array antenna greatly affects the scale and price of the device, it is difficult to apply the array antenna to small portable terminals.    Non-Patent Document 1: S. Haykin, “Cognitive Radio: Brain-Empowered Wireless Communications,” IEEE JSAC, VOL 23. NO.2. pp. 201-220, February 2005.    Non-Patent Document 2: Kikuma Nobuyoshi, “Adaptation Signal Processing Using Array Antenna,” Chap 6, Science and Technology Publishing, 1998.    Non-Patent Document 3: Winters, J. H., “Smart antennas for wireless systems,” IEEE Wireless Communications, Vol. 5, Issue. 1 pp. 23-27, February 1998.